feat: replace 'No calendars found' with 'Request Access to Calendar' button in onboarding

[devin-ai-integration]

Summary

In the onboarding Calendar tab, when the user hasn't granted calendar permission yet, the app previously showed an empty CalendarSelection with "No calendars found." This replaces that empty state with a dedicated "Request Access to Calendar" button.

• Adds RequestCalendarAccess component with a calendar icon and outline button
• Checks usePermission("calendar") status in CalendarSection
• Only renders the calendar list (AppleCalendarList) when permission is authorized; otherwise renders the request button

Review & Testing Checklist for Human

• UX: Redundant permission UI? — ApplePermissions (the red "Calendar" row with arrow button) is still rendered above the new RequestCalendarAccess button. Both trigger calendar.request. Verify this is the intended design, or if ApplePermissions should be hidden/simplified when not yet authorized.
• Test on macOS: Click "Request Access to Calendar" → system permission dialog appears → grant access → calendar list loads in place of the button.
• Test deny flow: Deny calendar permission → verify the request button remains visible and functional (re-requesting or opening settings).

Notes

• The generic CalendarSelection "No calendars found" empty state is unchanged — it just won't be reachable during onboarding anymore since we gate on permission status first.
• usePermission("calendar") is called unconditionally in CalendarSection even on non-macOS, but the result is only used inside the isMacos branch, so no functional impact.

Requested by: @goranmoomin
Link to Devin run

[devin-ai-integration]

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